A crankshaft miller of a type in which a milling cutter with the inner teeth being contacted with the outer surface of a work rotates at a high speed, thereby machining the journal of a crankshaft to have a cylindrical circumference, is known. A motion of the milling cutter is shown in FIGS. 14(a) to 14(d) in which the center c of the milling cutter 1 moves in the direction of an arrow d along a path (circle) b located at a predetermined distance r from the center a of a work WK. A plurality of tips 1a formed on the inner side of the milling cutter 1 rotates about the center c, thereby cutting the outer surface of the work.
The milling cutter 1 is moved under the control of a program prepared by an NC machine. In the NC program, the motion of the milling cutter is defined by a single command given by, for example,
G02 X90. Y75. I0. J35.
In the command, "G02" indicates "arc interpolation designation", "X90. Y75." indicates "X and Y coordinates at the start point S and the end point E (same as the start point S) of the center c of the milling cutter 1" and "I0. J35." indicates "X and Y coordinates of the rotational center a relative to the start point S".
In machining the outer surface of the work WK such as a crankshaft, rigidity of the work varies with the position of the rotation. Accordingly, roundness of the circumference of the machined work WK varies as indicated by a contour C' in FIG. 15, which is deformed from a circle C, causing an error. To make the contour C' of the work WK coincident with the circle C by displacing inward or outward the segments e, f, g, and h of the contour C', the path through which the center of the milling cutter 1 moves must be corrected into a path b' having segments e', f', g', and h' as shown in FIG. 16 according to the deformation of the segments e, f, g, and h.
As mentioned above, commands used in an NC machine are such a command that "move along a circle of a fixed radius". The correction that is carried out by the command is limited to a correction of the milling cutter motion over the entire circle. It is impossible to correct each segment of the contour indicated by the path b'. With such a command, it is impossible to improve roundness of the contour of a machined work.
The present invention has been made in view of the above background and its object is to provide a machining apparatus in which a path of its cutting tool is corrected according to the commands different for each segment of the circumference of a work, thereby remarkably improving roundness of the resultant product.